Regulated voltage generator for integrated circuit

ABSTRACT

A regulated voltage generator provides different regulated voltages to an integrated circuit. The regulated voltage generator includes a bandgap reference circuit and at least one gain stage connected to an output thereof. The output voltage of the bandgap reference circuit varies as a function of temperature to compensate for variations in the gain stage made up of first and second transistors. A regulated voltage output by the regulated voltage generator is independent of temperature and of the supply voltage. The value of the regulated voltage is adjusted via a load resistor and via the first and second transistors along with an output transistor of the bandgap reference circuit.

FIELD OF THE INVENTION

[0001] The present invention relates to integrated circuits, and moreparticularly, to voltage generators which provide different referencevoltages required for supplying integrated circuits.

BACKGROUND OF THE INVENTION

[0002] External power supplies for integrated circuits now vary betweenthree volts and ten volts, whereas the voltages required by the internalpower supplies for the electrical circuits within the integratedcircuits are, depending on the application, 2.5 volts, 3 volts, 5 voltsand 7 volts. These voltages are within ± 10%. It is therefore imperativethat an integrated circuit itself generate these different voltages inorder that they be independent of the power supply voltage and oftemperature. For instance, the temperature may vary between −40° C. and125° C.

[0003] To this end, there has been proposed a regulated voltagegenerator which exploits the properties of a reference voltage given bya circuit described in an article by E. Vittoz and J. Fellrath, entitled“CMOS Analog Integrated Circuits Based on Weak Inversion Operation”,published in IEEE Journal of Solid State Circuits, Vol. SC-12, no. 3,1997, pages 224-231. This voltage reference circuit is generally knownas a bandgap voltage reference circuit.

[0004] This prior art circuit supplies a reference voltage of 1.28volts, known as the bandgap voltage, which is constant over a wide rangeof supply voltages and temperatures. To obtain the different requiredvoltages, the circuit's output voltage is applied to gain stages, whicheach gain stage producing one of the required voltages.

[0005] However, these gain stages are sensitive to the supply voltageand to temperature, and the same holds for the power stage that followsthem for supplying the required power. As a result, the voltagessupplied vary significantly as a function of power supply voltage and ofthe temperature.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide a generator forat least one regulated voltage that is not very sensitive to variationsover a wide range of power supply voltages and temperatures.

[0007] This object is achieved by using a potential barrier referencevoltage circuit, known as a bandgap type of circuit, and at least onegain stage. To provide a regulated voltage generator that is notsensitive to variations in the power supply voltage and temperature, thecharacteristics of the reference voltage are degraded to compensate forthe variations due to the gain stage. The reference voltage thendelivers a voltage which is a function of temperature variationsopposite to that of the gain stage.

[0008] Another object of the present invention is to provide a generatorproducing a plurality of regulated voltages by implementing several gainstages.

[0009] The invention thus relates to a regulated voltage generator forsupplying at least one regulated voltage to an integrated circuitcomprising a bandgap type of reference voltage circuit and at least onegain stage. The bandgap type of reference voltage circuit comprises acurrent generator which supplies a bipolar transistor configured as adiode via a load resistor connected to the emitter of the bipolartransistor.

[0010] The gain stage comprises two MOS transistors in series betweenthe supply voltage and a ground potential. The gate of a firsttransistor is connected to the gate of the output transistor of thecurrent generator, and the gate of the second transistor is connected tothe output of the bandgap type reference voltage circuit.

[0011] The characteristics of the first and second transistors arechosen to obtain the regulated voltage. The value of the load resistoris chosen such that the emitter-base voltage of the bipolar transistorvaries with temperature in a manner to compensate for the variation ofthe gate-source voltage of the second transistor as a function oftemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Other characteristics and advantages of the present inventionshall become more apparent from reading the following description of thepreferred embodiments, given with reference to the appended drawings inwhich:

[0013]FIG. 1 is a schematic circuit diagram of a regulated voltagegenerator in accordance with the present invention; and

[0014]FIG. 2 is a block diagram of a device which delivers a regulatedvoltage among several available voltages in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The regulated voltage generator 10 in accordance with theinvention comprises (FIG. 1) a bandgap (potential barrier) referencevoltage circuit 12 and at least one gain stage 14. The circuit 12comprises four transistors M1, M2, M3 and M4 which are connected in aclosed loop.

[0016] Transistors M1 and M2 are N-type MOS transistors whose sourcesare connected to a terminal at ground potential GND, either directly fortransistor M2, or via a resistor R for transistor M1. The gates oftransistors M1 and M2 are connected to one another and to the drain oftransistor M2, which is connected to the drain of MOS transistor M4.Transistor M4 is a P-type transistor, and its source is connected to thesupply voltage V_(PS). The gate of transistor M4 is connected to thegate and to the drain of MOS transistor M3, which is a P-typetransistor, and is connected to the drain of transistor M1. The sourceof transistor M3 is connected to the supply voltage V_(PS).

[0017] The gates of transistors M3 and M4 are connected to the gate of aP-type MOS transistor M5 whose source is connected to the supply voltageV_(PS). The drain of transistor M5 is connected to the ground potentialGND via a resistor R₂, and a PNP type bipolar transistor Q1 is connectedas a diode. Bipolar transistor Q1 has its emitter connected to aterminal of resistor R₂ while its other two electrodes are connected tothe ground potential GND so that it functions as a diode.

[0018] The bandgap type reference voltage circuit 12 has two outputterminals 16 and 18. One output terminal 16 corresponds to the commonnode of the gates of transistors M3, M4 and MS, and the other outputterminal 18 corresponds to the drain of transistor M5.

[0019] The gain stage 14 comprises two P-type MOS transistors M6 and M7.The gate of transistor M6 is connected to output terminal 16, while thegate of transistor M7 is connected to output terminal 18. The source oftransistor M6 is connected to the supply voltage V_(PS), while its drainis connected to the source of transistor M7. The drain of transistor M7is connected to the ground potential GND. The regulated output voltageV_(G2) is taken from the terminals of transistor M7, i.e., between theground potential GND and the source of transistor M7.

[0020] Transistors M1 to M5 and resistor R form a current sourceproducing a current I_(GT). This current is supplied by transistor M5,and flows through resistor R₂ and bipolar transistor Q1. Transistor Q1is connected as a PN diode, and the current I_(GT) varies proportionallywith temperature.

[0021] In a prior art bandgap type of reference voltage circuit, thevalue of R₂ is chosen to produce a voltage V_(GAP)≈1.28 volts at theterminals of Q1 and R₂, which is not sensitive to temperature. Thisvoltage V_(GAP) is used in the gain stage 14 to obtain the requiredvoltage V_(G2), which is greater than V_(GAP).

[0022] In this gain stage, since the output voltage V_(G2) is the sum ofV_(GAP) and the voltage V_(SG7) between the gate and the source oftransistor M7, with V_(SG7) varying with temperature, V_(G2) also varieswith temperature.

[0023] The invention includes making V_(GAP) vary, so that it becomesV*_(GAP), as a function of temperature in order to compensate for thevariation of V_(SG7) as a function of temperature. This is obtained byadjusting the value of resistor R₂ and the sizes of transistors M5, M6and M7.

[0024] To this end, a first equation defines the current I_(GT):

I _(GT)(T)≈I _(GT)(T ₀)×(T/T ₀)  (1)

[0025] with the temperature T being expressed as an absolute value, andthe temperature T₀ being the reference temperature of 27° C.

[0026] A second equation defines the output voltage V_(G2) such that:

V _(G2) =V* _(GAP) +V _(SG7) ≈V _(EB) +R ₂ I _(GT) +V _(T7)+η₂ {squareroot}I _(GT)  (2)

[0027] where

[0028] V_(EB) is the emitter-base voltage of transistor Q1,

[0029] η₂ is a term which depends on the W/L `coefficients oftransistors M5, M6 and M7,

[0030] V_(T7) is an intrinsic characteristic voltage of transistor M7,referred to as the threshold voltage, and

[0031] V*_(GAP) is the variable voltage which depends on the temperatureat the terminals of resistor R₂ and of bipolar transistor Q₁. This isthe output voltage of the bandgap reference voltage stage.

[0032] A third equation defines the variation of η₂ as a function oftemperature:

η₂(T)≈η₂(T ₀) (T ₀ /T)^(m)  (3)

[0033] with m being in the region of two.

[0034] These three equations (1), (2) and (3) make it possible todetermine the values of η₂ and R₂ by the following formulas:

η₂≈0.4[(V _(G2) −V _(EB) +V _(T7))−T ₀(δV _(EB) /δT)]/[{square root}I_(GT)(T ₀)]  (4a)

R ₂=0.2[3(V _(G2) −V _(EB) +V _(T7))+2T ₀(δV _(EB) /δT)]/[I _(GT)(T₀)]  (4b)

[0035] with δV_(EB)/δT being in the region of 1.8 mV/° C.

[0036] These two formulas lead to values of R₂=550 kΩ and η₂=493 toobtain a value V_(G2)=2.94 volts, which varies by 300 μV/° C., that is49.5 mV in the temperature range of −40° C. to +125° C. for V_(PS)=10volts.

[0037] The voltage V*_(GAP) can be used to obtain other voltages V_(G1)and V_(G3) by applying that voltage to two gain stages 14′and 14″inwhich the transistors M′6, M′7 and M″6, M″7 are determined by thecoefficients η₁ and η₃ calculated using formula (4a). Calculated valuesof η₂=493 for V_(G1)=2.46 volts and η₃=635 for V_(G3)=3.43 volts areprovided, for example.

[0038] However, these voltages V_(G1) and V_(G3) are sensitive totemperature variations, on the order of a millivolt per degree Celsius.To obtain a voltage V_(G1) or V_(G3) that would not be sensitive totemperature, it would be necessary to modify R₂ according to formula(4b) to obtain R₁ for the case of voltage V_(G1), and R₃ for the case ofvoltage VG₃.

[0039] Moreover, coefficient η₂ not only determines the characteristicsof transistors M6 and M7, but also transistor of M5 according to theformula:$\eta_{2} = \frac{\lbrack {{W6} \cdot {{L5}/{W5}} \cdot {L6}} \rbrack^{1/2}}{\lbrack {{\mu 7} \cdot {{Cox}( {{W7}/{L7}} )}} \rbrack^{1/2}}$

[0040] where:

[0041] W and L are respectively the width W and the length L of thedrain-source channel of transistors M5 (W5 and L5), M6 (W6, L6) and M7(W7, L7), μ7 is the mobility of transistor M7, and Cox is the oxidecapacitance.

[0042]FIG. 2 is a functional block diagram of a device which suppliesone of the three voltages V_(G1), V_(G2) or V_(G3) on demand. Thisdevice comprises the bandgap type reference voltage circuit 12 of thediagram in FIG. 1, and supplies on output terminal 18 the voltageV*_(GAP) as well as the voltage V_(GT) of transistor M5 on outputterminal 16. Output terminals 16 and 18 are connected to the inputterminals of the gain stages 14′, 14 and 14″, which respectively supplythe voltages V_(G1), V_(G2) and V_(G3).

[0043] Only the voltage V_(G2) which corresponds to the value R₂calculated from formula (4b) is in fact regulated, and hencesubstantially independent of temperature variations. The outputterminals of gain stages 14′, 14 and 14″ are each connected to one ofthree input terminals 22, 24, 26 of a multiplexing circuit 30 whichproduces the connection between one of the three input terminals 22, 24,26 and its output terminal 28. Selection of the connection is obtainedby a control circuit 32 using appropriate signals.

[0044] The output terminal 28 of the multiplexing circuit 30 isconnected to the input terminal of a power amplifier 34, whose outputterminal 36 is connected to an electronic circuit to be supplied, suchas a microprocessor 38, for example.

That which is claimed is:
 1. Regulated voltage generator for supplyingat least one regulated voltage (V_(G1), V_(G2), V_(G3)) to an integratedcircuit (38), comprising a bandgap type of reference voltage circuit(12) and at least one gain stage (14, 14′, 14″), said bandgap type ofreference voltage circuit (12) comprising a current generator (M1 to M5,R) which supplies a bipolar transistor (Q₁) connected as a diode via aload resistor (R₂) connected to the emitter of the bipolar transistor(Q₁), characterized in that: the gain stage (14, 14′, 14″) comprises twoMOS transistors (M6, M7) in series between the supply voltage (V_(PS))and the ground terminal (GND), the gate of a first transistor (M6) beingconnected to the gate of the output transistor (M5) of the currentgenerator and the gate of the second transistor (M7) being connected tothe output of the bandgap type reference voltage circuit, thecharacteristics of said transistors (M6, M7) being chosen to obtain theregulated voltage (V_(G1), V_(G2), V_(G3)), and the value of the loadresistor (R₂) is chosen such that the emitter-base voltage (V_(EB)) ofthe bipolar transistor (Q1) varies with temperature in a manner tocompensate the variation of the gate-source voltage (V_(SG7)) of thesecond transistor (M7) as a function of temperature.
 2. Generatoraccording to claim 1, characterized in that the characteristics of thetwo transistors (M6, M7) of the gain stage are defined by the formula:η₂≈0.4[(V _(G2) −V _(EB) +V _(T7))−T ₀(δV _(EB) /δT)]/[{square root}I_(GT)(T ₀)]  (4a) in which formula: V_(G2) is the value of the regulatedvoltage to be obtained, V_(EB) is the emitter-base voltage of thebipolar transistor (Q₁), V_(T7) is the threshold voltage of transistorM7, T₀ is the reference temperature, I_(GT) is the current supplied bythe current generator, and δV_(EB)/δT is the variation of the voltageV_(EB) as a function of temperature T.
 3. Generator according to claim 1or 2, characterized in that the load resistor (R₂) is defined by theformula: R ₂=0.2[3(V _(G2) −V _(EB) +V _(T7))+2T ₀(δV _(EB) /δT)]/[I_(GT)(T ₀)]  (4b)
 4. Generator according to any one of claims 1 to 3,characterized in that it further comprises a multiplexing circuit (30)to which are applied the voltages (V_(G1), V_(G2), V_(G3)) and which iscontrolled by a control circuit (32) such as to select one of saidvoltages (V_(G1), V_(G2), V_(G3)).
 5. Generator according to claim 4,characterized in that it further comprises a power amplifier (34) towhich is applied the voltage selected by the control circuit (32).